Question

Isotopes differ due to the number of what?

Answer 1

...due to the number of NEUTRONS....

Explanation:

Isotopes differ with respect to the number of NEUTRONS their nuclei contain, where neutrons are massive particles of ZERO charge...

A nucleus may contain VARIABLE numbers of neutrons, and this gives rise to the phenomenon of isotopes...

And isotopes are best illustrated by their occurrence in the simplest (and most abundant) element, hydrogen. All hydrogen nuclei contain ONE nuclear proton, i.e. `Z=1`; most hydrogen nuclei CONTAIN only this proton to give the protium isotope, `""^1H`; a few hydrogen nuclei contain ONE neutron, to give the deuterium isotope, `""^2H`; and fewer hydrogen nuclei still contain TWO neutrons to give the tritium isotope, `""^3H`. Such isotopes are very useful in so-called labelling experiments, and also given that the isotopes are fairly cheap.

The heavier elements, especially the transition metals, typically have NON-integral atomic masses due to their existence as an envelope of isotopes. The atomic mass of iron given on the Periodic Table is `55.85*g*mol^-1`... The percentage abundance of the common iron isotopes are `""^54Fe`, `5.85%`, `""^56Fe`, `91.75%`, and `""^57Fe`, `2.12%`, `""^58Fe`, `0.28%`...the weighted average of these isotopes are of course...`55.85*g*mol^-1`.